To calculate the proportion below 70,

=
(70 - 60)/10 = 1

A z table can be used to
determine that the proportion of scores less than 1 standard
deviation above the mean is 0.8413. So, if 0.1587 of the
scores are above 70 and 0.0228 are above 80, then the proportion between
70 and 80 can be computed by subtracting 0.0228 from 0.1587: